AWS::SSM::Parameter - New Return Value

AWS::SSM:Parameter has the parameter name as the Ref output, but there is no build in way to get the Arn (see also aws-cloudformation/cloudformation-coverage-roadmap#68). Which means that you have to build the Arn yourself every time (eg. in IAM policies).

Current way to get an Arn
```
!Sub "arn:aws:ssm:${{AWS::Region}}:${{AWS::AccountId}}:parameter${MyParameterResource}"
```

Preferred way to get the Arn:

```
!GetAtt MyParameterResource.Arn
```

Category: Management & Governance

Review the AWS::SSM::Parameter resource behavior and the related cloudformation-coverage-roadmap#68 reference first. No repository files, tests, or implementation entry points are named; done would mean the resource exposes its ARN through GetAtt as requested.

Written by the indexing model from the issue text.